Field of the Invention
The invention related to a circuit arrangement for processing a video signal.
Processing a video signal in a television receiver is subjected to various conditions by which a faultless reproduction of the picture contents of the video signal in a picture display device, for example a display tube, is to be ensured. One of the measures of ensuring a correct display is to clamp a fixed black level in the video signal at a predetermined value. By means of this clamping, a DC level, which is not transmitted from the transmitter to the television receiver, is introduced into the video signal in such a way that a correct separation of synchronizing signals contained in the video signal is possible, and that it is used for adjusting the signal levels required for controlling the picture display device, particularly for adaptation to the cut-off point of the display tube. It is also known to control the amplitude of the video signal so that the picture contents can be displayed with the correct brightness. Since the amplitude control of the video signal is to be independent of the picture contents, to avoid fluctuations in the control of brightness and contrast of the displayed picture with changes in the picture contents, the control information is only derived from the synchronizing signals. In conjunction with the operation of clamping the video signal, a predetermined white peak value is then determined for the video signal which assumes this value at those areas in the picture that represent the whitest picture contents to be displayed.
However, the blackest parts of the picture contents may assume signal values which deviate from the fixed black level to an unforeseeable extent. If the dark control of the picture display device, for example, the cut-off point of the display tube, is adjusted only at the fixed black level (also referred to as black porch) in the video signal, inadmissible bright parts are also obtained in the displayed picture in parts which are to be displayed as black pans. For a correct picture display, it is therefore also necessary to correct the video signal to such an extent that the blackest picture parts are always displayed as black pans, but that also the brightest picture parts are always displayed as bright or white parts.